1. Field of the Invention
This invention relates to antennas, and more particularly relates to frequency independent circular antenna arrays.
2. Description of the Prior Art
Electronically steerable circular array antenna systems are well known in the art and are described in numerous patents, such as, for example, U.S. Pat. No. 4,414,550 to Carl P. Tresselt and U.S. Pat. No. 4,316,192 to Joseph H. Acoraci. Such circular arrays have N number of antenna elements, and are usually coupled to an N.times.N Butler matrix, N or N-1 phase shifters and a signal combining network. As is well known, the Butler matrix is an orthogonal beam network that provides a discrete Fourier transform of the signals received by the antenna elements. The transformed signals have amplitudes which are substantially independent of the direction of wavefront incidence and phase values which are approximately linearly dependent on direction of wavefront incidence. In these respects, the transformed signals resemble those produced by a linear array, so it is said in the art that the Butler matrix "linearizes" the circular array. Indeed, the plurality of phase shifters which are used to steer the antenna beam pattern and the signal combining network which is used to form the beam pattern of the circular array are interconnected and controlled in a manner similar to that for a linear array.
One of the problems with Butler matrix fed circular arrays producing electronically steerable directional beams is that they tend to have limited bandwidths. It has been discovered that for frequencies where the spacing between the antenna elements of the array is in excess of about 0.5 wavelengths, beam distortion and scanning irregularities increase rapidly with an increase in spacing. The reason is that the approximation used to linearize the circular array is accurate only when the spacing between antenna elements is small, and the spacing must generally be less than about one-half wavelength for the approximation to hold true.
Accordingly, one would like to keep the spacing between antenna elements low. However, it has been discovered that for spacings less than 0.3 wavelengths, mutual coupling between antenna elements causes impedance mismatch which increases with a decrease in spacing.